As optical networks move to higher speed modulation, such as 10 or 40 Gb/s, PMD is recognized as a source of signal distortion and increased bit error rates which may result in system outages. PMD causes pulse broadening, eventually making individual data bits indistinguishable. PMD varies widely with wavelength, and therefore compensation at one wavelength may actually increase PMD at another wavelength. Therefore, PMD detection must be wavelength specific, and compensation must be channel specific.
Known compensators typically measure the distortion, for instance by measuring the degree of polarization (DOP) of a single channel and provide compensation for the single channel. Although this approach works well, it requires a complex compensator and monitor for each channel, resulting in high cost for the overall transmission system.
A simplified broad band PMD monitor is still highly desirable to make high speed transmission possible.
A broad band system for measuring state of polarization (SOP) is disclosed in U.S. patent application publication No. 2002/0140943 by Dipakbin Chowdhury et al. published Oct. 3, 2002. The system discloses a tunable local oscillator which is operatively connected to a polarization transformer for transforming the local optical signal to a selected one of a plurality of polarization states. The polarized local optical signal is then combined with the transmitted optical signal and directed to a heterodyne detector to detect the beat frequency for determining the SOP of the transmitted optical signal. For each selected wavelength of the tunable local oscillator, three different polarization states are measured, prior to scanning to a next selected wavelength. This process is both slow and complex, requiring two degrees of control to the local oscillator, wavelength and polarization state. The polarization controller is expensive and relatively slow. The slow response is compounded by the need to obtain sequentially measurement of three different polarization states for each wavelength.